Microporous sheets have been conventionally used as moisture-permeable materials for moisture-permeable waterproof fabrics. Known examples of such microporous sheets are those formed by stretching a polytetrafluoroethylene resin to form a porous sheet or polyurethane resin films produced by wet film formation technique (see, for example, Patent Literature 1). Unfortunately, conventional porous sheets reduce the moisture permeability due to clogging with sweat, dirt, or other substances, or fail to exhibit sufficient waterproofness.
In order to overcome those disadvantages, non-porous sheets coated with hydrophilic moisture-permeable polyurethane resins have been proposed (see, for example, Patent Literatures 2 to 4). These moisture-permeable polyurethane resins contain polyol components such as polyoxyethylene glycol, which is a hydrophilic segment, and block copolymers of polyoxyethylene and polyoxypropylene.
Those non-porous sheets have a dramatically higher moisture permeability when they are coated with a thinner polyurethane resin film. However, those sheets have low strength due to low strength of resin and fail to exhibit sufficient waterproofness. Moreover, since the resin has a high permanent set, those sheets have insufficient resistance to washing exemplified by plastic deformation of resin resulting in wrinkles after washing.